This invention relates to printing machines for printing on individual sheets with different printing plates of different thickness, and more particularly, to a printing machine having an adjustable feed mechanism for feeding the sheets with variable timing so as to compensate for printing plates of different thicknesses.
As shown by way of example in U.S. Pat. Nos. 4,867,433 and 5,074,539, it is known to successively feed individual sheets of material, such as corrugated cardboard, into the first stage of a printing machine by means of feed belts which engage each sheet and then accelerate each sheet toward the printing stage; said U.S. patents being hereby incorporated by reference. Such feeding systems perform an excellent function of feeding one or two sheets per machine cycle with excellent registry of each sheet with the print plate. This produces very high quality multiple-color images on sheets, such as sheets to become containers which are generally known as container blanks.
More recently, however, it has become possible and desirable to use print plates of much less thickness than the older print plates, and the thinner print plates have their own advantages. The problem is-that it is not economic to throw away the older, thicker print plates when they still have a significant wear-life left. As a result, the same printing cylinders are sometimes fitted with the older, thicker plates and sometimes fitted with the newer, thinner plates. This creates a serious problem in that the difference thicknesses of the plates increases or decreases the combined diameter of the cylinder and associated plate. This means that the critical registry of the sheet and the rotary position of the print cylinder is changed depending upon whether the print cylinder is fitted with a relatively thick or thin printing plate, and this decreases the quality of the multi-color image which is printed.
The present invention solves this serious problem by varying the feed timing so that each sheet is delivered to the rotary print cylinder at precisely the correct instant so as to correct or compensate for variations in the thickness of the print plate.